Dispersions have long been known to strengthen various alloys and there are a variety of methods for forming dispersion-strengthened alloys. The methods typically use powder metallurgy. Early work in dispersion-strengthened alloys produced thoria-dispersed nickel via precipitation of powders from aqueous solutions. Another early alloy, sintered aluminum power, made dispersion-strengthened aluminum alloys from a slightly oxidized fine aluminum powder.
Internal oxidation is another technique for producing dispersion-strengthened alloys. Oxide dispersion-strengthened platinum and silver are commercially produced from solutions of the noble metal and a reactive element such as aluminum or zirconium. Oxidation and compaction results in a noble metal with a dispersed oxide such as alumina or zirconia.
Commercially produced dispersion-strengthened alloys, particularly nickel and aluminum alloys, have also been made by mechanical alloying. In this process, powdered forms of the matrix and dispersoids are mixed in a ball mill, wherein the balls pound the particulates thin and then weld them together, thereby mixing the dispersoids into the alloy matrix. Mechanical alloying of copper has not been commercialized, perhaps due to the fact that copper welds easily and adequate size reduction has not been obtained.
Another, more recent, method of making dispersion-strengthened metal alloys involves rapid solidification of alloys from a melt. Rapid solidification has been found to produce extremely fine microstructures in metals. In this technique, dispersion-forming constituents are dissolved in a molten alloy. During rapid solidification, these constituents precipitate as fine uniformly dispersed particulates within the alloy. Examples of the few thermally stable dispersion-strengthened alloys prepared by rapid solidification include FeAl+TiB.sub.2 (U.S. Pat. No. 4,419,130) and Al--Fe--Ce (See J. L. Walter et al., Eds. Alloying, ASM Int'l., p. 193, 1988). Many other systems have looked promising, but have proven unstable, primarily due to diffusion of the dispersion-forming elements at elevated temperature. Rapidly solidified dispersion-strengthened copper alloys have been studied by Sarin and Grant. See V. K. Sarin et al., Met. Trans., Vol. 3, pp. 875-878, 1972; and V. K. Sarin et al., Powder Metallurgy Int'l., Vol. 11, No. 4, pp. 153-157, 1979. The dispersions contained in these alloys were formed by reaction between reactive additives (chromium and zirconium) and oxygen contamination. All references cited herein are incorporated by reference as if set forth in full below.
Thus far, only internal oxidation has found commercialized use in a method for producing dispersion-strengthened copper alloys. One such alloy is "GLIDCOP" which is available from SCM Metal Products, Inc. This alloy contains finely dispersed aluminum oxide particles that are produced by internal oxidation. This alloy exhibits very high strength capabilities, but it is difficult to make cleanly, since the copper itself is partially oxidized. To remove the oxidized copper, the powder must be reduced after the internal oxidation step. This is difficult to do uniformly, since some copper oxide can become entrapped and hence will not outgas when reduced. Articles made from alumina dispersed copper alloys have exhibited some undesirable properties, including: microstructural inhomogeneity, reactivity with hydrogen, and hot shortness (brittleness at high temperature).